1. Field of the Invention
The present invention relates to a bonded optical disk used to record or play back audio/video information or general data optically, and a mold for molding base disks composing the bonded optical disk.
2. Description of the Prior Art
Bonded optical disks such as DVDs which are comprised of two base disks bonded together are well known today. As DVDs, so-called `single sided single layer` type bonded optical disk and so-called `double sided single layer` type bonded optical disk are known.
The `single sided single layer` type disk is comprised of a recording base disk for recording information and a dummy base disk for printing a label, both of which being bonded together, and recorded information on the recording base disk is read out from one side i.e. the surface of the recording base disk.
The `double sided single layer` type disk is comprised of two recording base disks bonded together, and recorded information on each recording base disk is read out from both sides of the bonded optical disk.
In order to produce a bonded optical disk such as a DVD, a transparent disk base of polycarbonate etc. with plenty of information pits is generally prepared first by injection molding.
For production of the `double sided single layer` type bonded optical disk, a single disk for the recording base disk of one side is prepared next by metallizing a reflection film of aluminum etc. on the surface of the pits of the molded disk base by sputtering etc. Then, as disclosed in JP-A-5-20714, ultra violet ray setting resin(UV-setting resin) is applied in the shape of a doughnut on the reflection film on the single disk, and another single disk is put upon this to bond them together. These single disks get bonded together by so-called spinner treatment in which the disks are spinned and the UV-setting adhesive spreads all over the surfaces of the disks. At this stage, ultraviolet rays are irradiated from one side or both sides of the bonded disk and the adhesive is hardened and set. Incidentally, for the `single sided single layer` type bonded optical disk, the reflection film is not needed to be metallized on the dummy base disk.
There exists another type of bonded optical disk which is so-called `single sided dual layer` type bonded optical disk. This disk is comprised of a transparent base disk on which semitransparent film is metallized and a recording base disk on which a reflection film of aluminum etc. is metallized, and both base disks are bonded together. Recorded information on two layers is read out form the side of the base disk with semitransparent film. This `single sided dual layer` type bonded optical disk is produced by similar process to the process described above.
For these types of bonded optical disks, a strict standard is established as to the `tilt` which indicates the warp angle of recording area from the held part of the disk (angle .alpha..ltoreq..+-.0.8.degree.).
However, the direction and the amount of the warp vary widely between base disks which are supposed to be bonded together, according to sectional structure and molding condition (residual stress etc.) of the base disks.
Since it is substantially impossible to bond two base disks having exactly the same amount and the opposite direction of warps, produced bonded optical disks are forced to have not a little `tilt` caused by the difference of the warps. Therefore, it has been impossible to steadily mass produce and supply bonded optical disks with minimum tilts.
Moreover, there exists another standard for the `single sided dual layer` type bonded optical disks as to the thickness of adhesive layer bonding two base disks (40-70 .mu.m, .+-.10 .mu.m within one disk, .+-.4 .mu.m within one revolution). Hence, in the conventional spinner application process, thickness of the adhesive layer has to be controlled by varying spinning speed and time according to the viscosity of UV-setting adhesive. However, the viscosity of this UV-setting adhesive tends to vary according to temperature and surface tension on base disks, requiring strict management of these parameters in the thickness control of the adhesive layer. Therefore, it has been impossible to steadily mass produce and supply bonded optical disks with uniform adhesive layer thickness between base disks.